The present invention relates to an anti-stall system which is utilized to reduce the displacement of a variable displacement hydraulic unit such as the pump of a hydrostatic transmission to reduce the drive ratio of the transmission, whereby overloading of a prime mover is prevented. More particularly, the invention relates to such a system having an electronic control and wherein, upon a decrease in the loading on the unit output, the control causes the displacement of the hydraulic unit to be increased at a slow rate to improve stability in operation of the prime mover.
Known anti-stall controls have been used to limit the power output from a hydrostatic transmission in which inputs from a prime mover, such as an engine, to a work stage are loaded by power outputs from the work stage. These anti-stall controls have generated an electrical signal which controls the hydrostatic transmission such that load torque on the engine is decreased in proportion to a decrease in the engine speed as sensed by the anti-stall control. Under dynamic conditions, rapid loading and unloading of the hydrostatic transmission by the known anti-stall controls may cause the engine to hunt, resulting in regenerative instability and decreased productivity.
Known anti-stall controls typically compare a signal representing the actual speed of the engine to a set point signal representing a desired engine speed to generate the electrical control signal. Heretofore the sensitivity of the control could not be adjusted without altering the set point, making it necessary to recalibrate the control whenever the sensitivity is altered.
An object of the invention is to provide a new and improved anti-stall electronic control system which controls the displacement of a variable displacement hydraulic pump to prevent overloading of a prime mover driving the pump and which provides a slow rate of change in pump displacement after the overload condition is removed to provide improved stability of the system.
Another object of the invention is to provide a new and improved electronic anti-stall control for a hydrostatic transmission which reduces the drive ratio of the transmission to prevent stalling of a prime mover upon an impending overload thereof and which increases the drive ratio at a slow controlled rate when the load drops to provide improved stability of the prime mover.
The anti-stall control includes means for sensing the speed of the engine to produce an actual speed voltage proportional thereto. The actual speed voltage is compared to a reference voltage for providing a D.C. signal having an amplitude proportional to the actual speed voltage. The D.C. signal is applied to a quick dump/slow recovery circuit, the output of which is a control voltage having an amplitude which decreases at a fast rate to a value proportional to the amplitude of the D.C. signal when the engine speed decreases and which increases at a slow rate to a value proportional to the amplitude of the D.C. signal when the engine speed increases.
The actual speed voltage is applied through a potentiometer to an op amp for comparison with respect to the reference voltage applied to another input of the op amp. The setting of the potentiometer establishes a set point which is the value of the reference voltage when the engine is operating at its optimum speed. A sensitivity potentiometer is provided in a feedback path connected between the output of the op amp and the reference voltage applied to the op amp so that the sensitivity of the anti-stall control may be varied without altering the set point. Thus, the control need not be recalibrated when the sensitivity is altered.
A nonlinear gain stage connected to the output of the quick dump/slow recovery circuit is provided so that the change in the control voltage when the engine is running at low speeds is more gradual than the change in the control voltage when the engine is running at high speeds.
A selectable attenuator is further provided so that the amplitude of the maximum control voltage generated by the anti-stall control may be varied depending on the work function being performed by the system.